Well I'm delighted to be able to report that at last I've got a workable solution to this issue. Very many thanks to all that offered suggestions, much appreciated.

I eventually got a working set up using a USB WiFi adaptor. Initially I was trying with an unbranded one - the driver loading software didn't work and when I came to remove the device, the metal bit of the adaptor remained in the USB socket and had to be extracted using fine needle point pliers

The break through came when I bought a 'named brand' that worked first time. To be in control of it's IP address and to prevent it discovering a gateway off my network I've had to give it a fixed IP address outside the DHCP range of my router - this lets me leave the gateway address blank. There is however a down side in that this PC cannot join my 'Homegroup', as Windows 7 INSISTS that members must use DHCP and if you kick off the Homegroup Troubleshooter blow me if it doesn't alter it to DHCP

As the fixed IP address is outside the range controlled by DHCP on my router I cannot use Parental Control time restrictions on this PC as was suggested as the router is unaware of the machine, however not having a gateway address is I think sufficient safety in this instance.

So my 'CNC Plasma Table PC', which is physically built into the table, is now operating on two networks. 192.168.1.xxx via the WiFi adaptor which is my home network, and 192.168.10.xxx via the inbuilt network controller which is the network that the Russian Pure Logic ethernet break out board and torch height controller use.

I can now transfer files generated in the warmth of the house or my heated workshop, into the (currently) freezing cold welding shop without having to take a Thumb drive (*)

(* which I was everlastingly leaving plugged in so the PC tried to boot off it !)

There is something distinctly odd about the way Win7 handles local networing and naming of devices on the network. As I said above, I can transfer files to shared directories I've 'opened up' on my plasma PC, which shows up by name under the local network using Windows Explorer (NOT Internet Explorer!) However, if I open up a 'command prompt' and ping the Plasma PC by name I get rapid responses, but saying that the replies are from the MAC address of the PC. If I ping it by IP address it uses the PC name for the address

Doesn't really matter as I can achieve what I originally wanted to do, but rather odd.

If I look at Windows Explorer on the Plasma PC and try and examine Home Groups, it says the machine is NOT a member of the home group, and as stated earlier if I run the Home Group troubleshooter it changes the IP address to a DHCP derived one.

However if I look at Home Groups from another PC on the network, the Plasma PC is there not only under Network BUT ALSO AS A MEMBER OF THE HOME GROUP I can open and read directories and files perfectly normally.

(But it looks as though I gave duff gen in the previous post - I was using the wrong PC name and IP address when pinging- you can't get the staff )

I ended up accepting that to get easy communications to the Plasma PC it had to be a member of the Homegroup, and hence getting it's IP address by DCHP.

However this allowed it to show up on the Router 'Parental Control' table, and I've been able to stop it having access to the internet that way. But even that wasn't entirely straight forwards

The router allows me to block access with start times and end times in half hour increments. But this meant that there is no way to stop at least one half hour slot of access, however once the time is set up, you THEN have access to another box to tick for permanent blocking

But the programmer neglected to bring the tick box up on the screen until the other time setting boxes had been completed and applied - so as you rightly say, an awful user interface.

It never ceases to amaze me how some programmers don't think about these things. I've been using 'Inkscape' a lot recently, so have got used to one idiosyncrasy it has - you kick the program off by double clicking it's screen icon, then ABSOLUTELY NOTHING happens for what seems ages as it loads itself up BUT PUTS NOTHING ON THE SCREEN to tell you it's loading. The delay is quite long enough for you to think perhaps you failed to click properly so do it again, then you get two instances of it running